Method and apparatus for joint equalization and decoding of multilevel codes

ABSTRACT

A method and apparatus are disclosed for joint equalization and decoding of multilevel codes, such as the MLT-3 code, which are transmitted over dispersive channels. The MLT-3 code is treated as a code generated by a finite-state machine using a trellis having state dependencies between the various states. A super trellis concatenates the MLT-3 trellis with a trellis representation of the channel. Joint equalization and decoding of the received signal can be performed using the super trellis. A sequence detector is disclosed that uses the super trellis or a corresponding reduced-state trellis to perform joint equalization and decoding of the received signal to decode the MLT-3 coded data bits. The sequence detector may be embodied using maximum likelihood sequence estimation that applies the optimum Viterbi algorithm or a reduced complexity sequence estimation method, such as the reduced-state sequence estimation (RSSE) algorithm.

FIELD OF THE INVENTION

The present invention relates generally to channel equalization anddecoding techniques, and more particularly, to methods and apparatus forperforming joint equalization and decoding of a multilevel code.

BACKGROUND OF THE INVENTION

The transmission rates for local area networks (LANs) that useunshielded twisted pair cabling have progressively increased from 10Megabits-per-second (Mbps) to 100 Mbps. Multilevel Threshold-3 (MLT-3)is a bandwidth efficient line code that is employed in one of the mostpopular data transmission schemes for LANs, i.e., 100 Base-TX Ethernet,which has a data rate of 100 Mbps. FIG. 1 illustrates the mapping ofdata bits to the corresponding output signals in accordance with theMLT-3 code. As shown in FIG. 1, data bits are encoded in accordance withthe MLT-3 code using three signal levels (+1, 0, −1), where a data bitwith a logic value of one (1) causes a transition of the MLT-3 signaland a data bit with a logic value of zero (0) leaves the MLT-3 signalunchanged. It is noted, however, that the MLT-3 signal sequences (1, 0,1), (1, −1), (−1, 0, −1) and (−1, 1) are not allowed according to theMLT-3 code.

When MLT-3 coded signals are transmitted through a dispersive channel,the received signals will be corrupted by intersymbol interference andnoise. A number of systems have been disclosed or suggested forprocessing MLT-3 coded signals, such as those described in U.S. Pat.Nos. 6,115,418 and 6,178,198. FIG. 2 illustrates a receiver architecture200 that is typical of such prior art receiver designs. As shown in FIG.2, the exemplary conventional receiver 200 includes an equalizer 210that cancels the intersymbol interference (ISI) in the received signal,r_(n). The equalizer 210 can be implemented as a linear equalizer or adecision-feedback equalizer, in a known manner. Thereafter, athree-level slicer 220 performs symbol-by-symbol detection to generateMLT-3 signal values of −1, 0 or 1. Finally, an MLT-3 decoder 250converts these detected MLT-3 signals into data bits, in accordance withthe mapping shown in FIG. 1.

While the receiver architecture 200 shown in FIG. 2 has relatively lowcomplexity, the receiver 200 suffers from a number of limitations, whichif overcome, could greatly improve the decoding of MLT-3 signals.Specifically, the receiver architecture 200 shown in FIG. 2 achievesonly sub-optimum performance as it performs symbol-by-symbol detection.Thus, conventional MLT-3 receivers, such as the receiver 200 shown inFIG. 2, exhibit unacceptable bit error rates in applications where thechannel impairments are severe, as is the case for high-speed datacommunications over copper. A need therefore exists for an improvedreceiver architecture for the detection of MLT-3 coded signals in thepresence of ISI and noise.

SUMMARY OF THE INVENTION

Generally, a method and apparatus are disclosed for joint equalizationand decoding of multilevel codes, such as the MLT-3 code. The presentinvention recognizes that the MLT-3 code can be treated as a codegenerated by a finite-state machine using a trellis having statedependencies between the various states. A super trellis is created thatconcatenates the MLT-3 trellis with a trellis representation of thechannel. Once the MLT-3 code and channel are represented as a supertrellis, joint equalization and decoding of the received signal can beperformed on the super trellis or a corresponding reduced-state trellis.A sequence detector is disclosed that uses the super trellis or thereduced-state trellis to perform joint equalization and decoding of thereceived signal to decode the MLT-3 transmitted data bits.

In one variation, the sequence detector is embodied as a maximumlikelihood sequence estimator (MLSE) that applies the optimum Viterbialgorithm to the super trellis. In a further variation, the sequencedetector can be embodied as a sequence estimation method with reducedcomplexity, such as the reduced-state sequence estimation (RSSE)algorithm or variants thereof.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional mapping of data bits to correspondingMLT-3 signals in accordance with the MLT-3 line code;

FIG. 2 illustrates a conventional receiver architecture for processingMLT-3 coded signals;

FIG. 3 illustrates an exemplary postcursor channel impulse response fora channel having a memory of length, L;

FIG. 4 illustrates a trellis that represents the various states of theMLT-3 code in accordance with the present invention;

FIG. 5 illustrates a trellis that represents a channel having a channelmemory of length one (L=1);

FIG. 6 illustrates a super trellis that concatenates the MLT-3 codetrellis of FIG. 4 with the channel trellis of FIG. 5;

FIG. 7 is a schematic block diagram illustrating a sequence detectorthat performs joint equalization and decoding of a received signal;

FIG. 8 is a schematic block diagram of a receiver in accordance with oneembodiment of the present invention where a Viterbi detector performsmaximum likelihood sequence estimation using the super trellis of FIG.6; and

FIG. 9 is a schematic block diagram of a receiver in accordance withanother embodiment of the present invention where a reduced-statesequence estimation algorithm performs joint equalization and decodingusing a reduced-state trellis corresponding to the super trellis of FIG.6.

DETAILED DESCRIPTION

The present invention recognizes that the MLT-3 code can be treated as acode generated by a finite-state machine having state dependenciesbetween the various states. Thus, according to one aspect of theinvention, the MLT-3 code is represented as a trellis and bit errorrates are thereby reduced for decoding MLT-3 coded signals in thepresence of channel impairments, such as ISI or noise. FIG. 3illustrates a postcursor channel impulse response 300 having a channelmemory of length, L. As shown in FIG. 3, the channel impulse response300 consists of a main tap corresponding to time 0, and L postcursortaps.

According to another aspect of the invention, joint equalization anddecoding of the received signal is performed on a super trellis thatconcatenates the MLT-3 code trellis with a channel trellis. In thismanner, a MLT-3-coded data sequence can be detected in the presence ofISI and noise using maximum likelihood sequence estimation (MLSE), whichapplies the Viterbi algorithm to the super trellis defined by theconcatenation of the MLT-3 code trellis and the channel trellis, or asequence estimation method with reduced complexity, such as thereduced-state sequence estimation (RSSE) algorithm or variants thereof.

Trellis Representation of the MLT-3 Code

FIG. 4 illustrates a trellis 400 that represents the various states,σ_(n), of the MLT-3 code. As shown in FIG. 4, the MLT-3 code trellis 400comprises four states, S0, S1, S2, and S3. State σ_(n)=S0 is associatedwith a previous MLT-3 signal value of +1. States σ_(n)=S1 and σ_(n)=S2are associated with a previous MLT-3 signal value of 0 (increasing anddecreasing transitions). State σ_(n)=S3 is associated with a previousMLT-3 signal value of −1. There are two branches leaving or enteringeach state corresponding to the state transitions associated with thedata bits 0 and 1, respectively. A data bit with the logic value 1causes a state transition, and a data bit with the logic value of 0 doesnot cause a state transition in the MLT-3 code trellis. For example, ifthe decoder is in State σ_(n)=S0 (the previous MLT-3 signal value was+1), and the current data bit to be encoded is a value of one (1), thedecoder will encode the signal as an MLT-3 signal value of 0 andtransition along the lower branch out of state σ_(n)=S0 to the nextstate σ_(n+1)=S1.

FIG. 5 illustrates a trellis 500 that represents a channel having achannel memory of length one (L=1), such as the channel shown in FIG. 3.In general, for a channel with memory L a channel state β_(n) can bedefined asβ_(n)=(b _(n−1) ,b _(n−2) ,b _(n−L)).

As shown in FIG. 5, the trellis 500 for the case that L=1 comprises twostates β_(n)=0 and β_(n)=1. State β_(n)=0 is associated with a previousdata bit value of zero (0). State β_(n)=1 is associated with a previousdata bit value of one (1).

FIG. 6 illustrates a super trellis 600 that concatenates the MLT-3 codetrellis 400 of FIG. 4 with the channel trellis 500 of FIG. 5. A combinedMLT-3/channel state in the super trellis is defined as follows:ρ_(n)=(σ_(n); β_(n))=(σ_(n) ; b _(n−1) ; b _(n−2) , . . . , b _(n−L))).It is noted that the number of states in the combined super trellis 600is 4×2^(L).

Sequence Estimation

As previously indicated, the present invention performs jointequalization and decoding of the received signal using the super trellis600 or a corresponding reduced-state trellis. FIG. 7 is a schematicblock diagram illustrating a sequence detector 710 that performs jointequalization and decoding of the received signal, r_(n), to generate thedetected data bits, b_(n). As previously indicated, the sequencedetector 710 may be embodied as a Viterbi detector 800, discussed belowin conjunction with FIG. 8, that performs maximum likelihood sequenceestimation on the super trellis 600 or a reduced-state sequenceestimation algorithm 900, discussed below in conjunction with FIG. 9, ora variation thereof. The minimum Euclidean distance between allowablesequences is 2 in the MLT-3 code trellis. Therefore, when there is noISI, the present invention allows for a decoding gain of 3 dB comparedto the above-mentioned conventional technique (see FIG. 2) where onlysimple symbol-by-symbol detection is being performed. In the presence ofISI, this gain might be even more significant.

FIG. 8 is a schematic block diagram of a receiver 800 in accordance withthe present invention, where a Viterbi detector performs maximumlikelihood sequence estimation on the super trellis 600. As shown inFIG. 8, the receiver 800 includes a branch metrics unit (BMU) 810, anadd-compare-select unit (ACSU) 820 and a survivor memory unit (SMU) 830.The BMU 810 calculates the branch metrics (BMs) for the transition inthe super trellis, the ACSU 820 performs the ACS operation for the supertrellis states, and the SMU 830 keeps track of the survivor paths.

Generally, the receiver 800 performs data detection using maximumlikelihood sequence estimation (MLSE), to produce data bits. In MLSE,all possible sequences are considered and it is determined whichsequence was actually transmitted, in a known manner. MLSE is theoptimum detection method and an efficient implementation is the Viterbialgorithm. For a more detailed discussion of implementation issuesassociated with the Viterbi algorithm, see, for example, GerhardFettweis and Heinrich Meyr, “High-Speed Parallel Viterbi DecodingAlgorithm and VLSI-Architecture,” IEEE Communication Magazine (May1991), incorporated by reference herein.

FIG. 9 is a schematic block diagram of a receiver 900 in accordance withthe present invention, where a reduced-state sequence estimation (RSSE)algorithm performs joint equalization and decoding on a reduced-statetrellis. Generally, the RSSE algorithm reduces the complexity of MLSE bymerging several super trellis states to produce a trellis with a reducednumber of states, that may be expressed as follows for the channel shownin FIG. 3:ρ_(n)′=(σ_(n); (b _(n−1) ,b _(n−2) , . . . b _(n−K))), K<L,where K is referred to as truncated channel memory. The number of statesin the reduced-state trellis is 4×2^(K). When K is chosen to be 0, thereduced-state trellis simplifies to the MLT-3 code trellis of FIG. 4. Inthis case, the reduced-state trellis itself does not contain anyinformation about the ISI, which is accounted for on a per-state basisby considering symbols from corresponding survivor paths. It is clearthat with an appropriate value for K a trade-off between decodingcomplexity and performance can be achieved. The closer K is to L, thebetter the bit error rate performance will be, but the decodingcomplexity will increase as well.

For a discussion of the RSSE algorithm, as well as the special cases ofdecision-feedback sequence estimation (DFSE) and paralleldecision-feedback decoding (PDFD) techniques, see, for example, P. R.Chevillat and E. Eleftheriou, “Decoding of Trellis-Encoded Signals inthe Presence of Intersymbol Interference and Noise”, IEEE Trans.Commun., vol. 37, 669-76, (July 1989), M. V. Eyuboglu and S. U. H.Qureshi, “Reduced-State Sequence Estimation For Coded Modulation OnIntersymbol Interference Channels”, IEEE JSAC, vol. 7, 989-95 (August1989), or A. Duel-Hallen and C. Heegard, “Delayed decision-feedbacksequence estimation,” IEEE Trans. Commun., vol. 37, pp. 428-436, May1989, each incorporated by reference herein. U.S. patent applicationSer. No. 09/326,785, filed Jun. 4, 1999 and entitled “Method andApparatus for Reducing the Computational Complexity and Relaxing theCritical Path of Reduced State Sequence Estimation (RSSE) Techniques,”incorporated by reference herein, discloses a modified RSSE techniquethat reduces the hardware complexity of the conventional RSSE algorithmfor a given number of states and also relaxes the critical path problem.

As shown in FIG. 9, the receiver 900 includes a branch metric units(BMU) that calculates the branch metrics based on the received signal,r_(n). The best surviving paths into reduced states are determined inthe add-compare-select unit (ACSU) and then stored in the survivormemory unit (SMU). The decision-feedback unit (DFU) takes the survivorsymbols from the SMU to calculate the ISI estimates for the reducedstates, which are used by the BMU to calculate the branch metrics forthe transitions in the reduced-state trellis. The receiver 900 can beembodied in a similar manner to the 1000BASE-T RSSE decoder described inE. F. Haratsch and K. Azadet, “A 1-Gb/s Joint Equalizer and TrellisDecoder for 1000BASE-T Gigabit Ethernet,” IEEE J. Solid-State Circuits,vol. 36, 374-384 (March 2001) or U.S. patent application Ser. No.09/471,920, filed Dec. 23, 1999, entitled “Method and Apparatus forShortening the Critical Path of Reduced Complexity Sequence EstimationTechniques,” each incorporated by reference herein.

In an alternate implementation, the reduced complexity sequenceestimator can employ the M algorithm (as opposed to RSSE) on the supertrellis of FIG. 6, where only the M paths with the best path metrics arekept at each decoding step. For a discussion of the M algorithm, see,for example, E. F. Haratsch, “High-Speed VLSI Implementation of ReducedComplexity Sequence Estimation Algorithms With Application to GigabitEthernet 1000 BaseT,” Int'l Symposium on VLSI Technology, Systems, andApplications, Taipei (June 1999), each incorporated by reference herein.

It is to be understood that the embodiments and variations shown anddescribed herein are merely illustrative of the principles of thisinvention and that various modifications may be implemented by thoseskilled in the art without departing from the scope and spirit of theinvention.

1. A method for decoding a signal received from a dispersive channelcausing intersymbol interference, said signal encoded using an MLT-3code, said method comprising the steps of: generating at least onetrellis representing both said MLT-3 code and said dispersive channel;and performing joint equalization and decoding of said received signalusing said trellis, wherein a state in said trellis is given by aconcatenation of said MLT-3 code state and a truncated channel state,wherein said truncated channel state partially describes said dispersivechannel, and wherein a number of states in said trellis is given by4×(2^(K)), where K is the truncated channel memory.
 2. A receiver forprocessing a signal received from a dispersive channel, said signalencoded using an MLT-3 code, comprising: a sequence detector thatperforms joint equalization and decoding of said received signal usingat least one trellis representing both said MLT-3 code and saiddispersive channel, wherein a state in said Trellis is given by aconcatenation of said MLT-3 code state and a truncated channel state,wherein said truncated channel state partially describes said dispersivechannel, and wherein a number of states in said trellis is given by4×(2^(K)), where K is the truncated channel memory.